1. Field of the Invention
The present invention relates to an image forming apparatus having an image reading device for correcting the density and color balance of an image to be formed on a recording medium.
2. Description of the Related Art
Conventionally, image forming apparatuses that use a variety of methods for forming an image on a recording medium on the basis of image data have been known.
However, such conventional image forming apparatuses, with any of the variety of methods, are not always able to provide the same printing results even when their driving conditions (e.g., driving voltage and driving time) are set to predetermined values. Specifically, dye-sublimation thermal printers may slightly vary in their printing characteristics depending on ribbon cartridges or roll paper cartridges being used, which would adversely affect the printing results. Further, the printing characteristics may also vary depending on the temperature or humidity when a ribbon cartridge or the like is stored or when printing is performed.
In order to suppress the aforementioned changes or variations in the printing characteristics and realize printing with stable image quality, the driving conditions in printing should be corrected in accordance with the printing conditions. To this end, there is disclosed a technique of printing test patterns in such a manner as to combine a plurality of patches whose color shades slightly differ from one another, on a gray background, visually judging the test patterns, and selecting a patch whose color is the closest to the background gray color. Then, correction is performed on the basis of the color components of the selected patch, whereby image quality is stabilized (see JP 2000-022954A).
There is also disclosed a technique of printing a test pattern with a halftone gray value, and measuring the amount of light reflected from the printed image using a photosensor, which is arranged downstream of a transport path of a recording medium, to determine the print density. Then, correction is performed on the basis of the difference between the determination result of the print density and print image data, whereby image quality is stabilized. Further, with this technique, it is possible to use, when measuring the amount of light reflected, a portion on which no test pattern is printed as a reference white (see JP 2000-326537A).
As described above, although there exists a technique of performing correction by reading a test pattern with a photosensor or the like, such a technique would also involve calibration of the photosensor. In the field of scanners, there is known a technique of providing a scanner calibration mode, reading a calibration sheet, which has been prepared in advance, with a scanner, and performing correction on the basis of the difference between the value of the image data on the calibration sheet and the reading result. With this technique, it is possible to correct shading, which results from luminance unevenness of a light source, by reading a white reference belt provided on the end of platen glass (see JP 2007-259123A).
Further, there is also known a technique of providing, in calibration of a color image scanner, a reference plate, which has a white region and a color patch region, in a region outside the document reading range. Such a technique performs calibration by reading the reference plate (see JP 2002-204371A).